Fiberoptics is the branch of physics concerned with the propagation of light that enters a thread or rod of transparent material at one end and is totally reflected back inward from the wall, thereby being transmitted within the fiber from one end to the other. Fiberoptics is widely applied in medical practice to observe the human body internally. Fiberoptic fibers have also been used to transmit light signals carrying information from both electronic and optical sensors.
The accurate measurement of temperature is important in many chemical processes to avoid harm to materials and equipment resulting from temperatures outside a specified range. Its determination is also necessary in situations in which it is a needed variable in the computation of properties, such as pressure, viscosity, or density. Methods of sensing temperatures depend upon the measurement of the changes caused by temperature. Many devices, such as the familiar glass-stem thermometer, measure the change in volume of a substance, such as mercury, caused by a change in temperature. Thermistors are devices that measure temperature by the change it causes in their electrical resistance. Temperature may be inferred by measuring the intensity of the total radiation emitted, as radiation pyrometers do, or by observing changes in color or shape of certain materials. These devices exist in such multiplicity to meet differing requirements of size, accuracy, range, and ability to withstand the testing environment.
One of the most versatile and widely used of these devices is the thermocouple. It operates on the principle that heat imparted to the junction of two different metals or alloys causes a voltage that varies with the amount of heat applied. The device consists of two wires of different metal, fused together at one end to form a measuring junction. The free ends are connected to a measuring instrument, which converts the voltage at the thermocouple junction into a measurement of the temperature, the two quantities being directly proportional.
Pressure, like temperature, is a variable that must be measured accurately in industry, particularly in the chemical industry. Determination of pressure is vital, for instance, in the control of hydrogenation (addition of hydrogen) and distillation in petroleum processing. Again, like temperature, pressure is a variable needed in the calculation of other properties. Pressure-measuring devices vary with the range over which they are meant to be used. In the vacuum range, gas pressure is detected by measuring the current generated due to ionization of the gas or by measuring the thermal conductivity of the rarified gas. Pressures in this region are also calculated by compressing a known volume of the gas until it reaches a fixed pressure. When the new volume is measured, the original pressure can be computed by use of Boyle's Law, which states that the product of the original volume and original pressure is equal to the product of the new volume and new pressure.
In the atmospheric pressure range and above, elastic pressure elements are widely used; they measure the expansion caused by pressure. While some devices measure the expansion of a diaphragm or a bellows, the most commonly used industrial sensor is the so-called Bourdon tube consisting of a tube in the form of a 250.degree. arc. The process pressure is connected to the fixed socket end of the tube, while the tip end is sealed and connected via a series of links and gears to a pointer. Because of the difference between its inside and outside radii, the Bourdon tube tends to straighten when pressure is applied. The resulting motion of the sealed tip is a function of this pressure, and thus, the position of the pointer yields a measure of the process pressure.
A device that has many applications in this pressure range is the strain gauge, which is based upon the fact that metallic conductors subjected to strain exhibited corresponding change in electrical resistance. There are many types of strain gauge, but all are constructed so that the process pressure causes a strain, and thus a change in electrical resistance, which is measured for a visual display. In one example, the process pressure is applied to a flat diaphragm. The strains resulting from the diaphragm deflection cause changes in the resistance of four strain elements bonded directly to the underside of the diaphragm. This change in resistance is measured as an indication of process pressure.
In many circumstances, particularly in the dangerous environment of chemical processing industries, it is necessary to measure pressure and temperature while avoiding any potential ignition of volatile gases in the area. Whenever electricity is directly applied in the measurement of temperature and pressure in such applications, there is an inherent risk that a short circuit or other electrical malfunction may occur that could ignite a dangerous mixture.
While fiberoptics have been used in the past for the measurement of temperature and pressure, these techniques have required the use of complicated bundles of optical fibers and electronics associated with those optical fibers. These arrangements have been costly, logistically difficult, unduly complicated, and generally unreliable. The present invention is believed to be the first application of a single optical fiber approach to the measurement of temperature and pressure.
It is an object of the present invention to provide an inherently safe technique for the measurement of temperature and pressure.
It is another object of the present invention to provide a fiberoptic sensor system that produces an analog output relative to the effect of temperature or pressure upon a sensor.
It is still another object of the present invention to provide a fiberoptic sensing system that stabilizes the source of light that is directed toward the optical fiber within such a system.
It is still another object of the present invention to provide a fiberoptic sensing system for the measurement of temperature or pressure that utilizes a single optical fiber for the transmission of light information to and from the sensing device.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.